Worms, Trojans, and viruses are programs created by hackers and are used to infect at risk computers via the Internet. Worms and viruses can replicate themselves from computer to computer, while Trojans hide in seemingly legitimate programs that you download from the Internet. The list of things that can go wrong with your computer is endless once your computer is infected with a virus, worm, or Trojan.
One of the first signs that you are indeed infected is pop up advertisements and a big drop in the performance or speed of your computer. Antivirus software scans your computer constantly looking for these bad guys, and if found either quarantines the malicious code or deletes it before it can do harm to your computer and data. Windows XP does not have a built in antivirus. An excellent free antivirus program that you can download from the Internet is AVG. Just Google this acronym for the download site, and you'll be set for a good antivirus tool. An alternative is to go to Microsoft's Antivirus Partners Webpage and see what they recommend. Microsoft also has a free program out called Defender that is supposed to be good protection from viruses and spyware.
Viruses represent a special challenge to detection because of the fact that they change on a daily or sometimes hourly basis. For this reason it is best to choose an antivirus program with an automatic update capability. The updates to the software are to keep abreast of the ever present change seen in the types of viruses found on the Internet. If your antivirus software is not updated your computer becomes vulnerable to these new threats.
To completely cover your bases against these types of threats to your computer, be sure and add a good firewall as well as a anti spyware program to your computer for the best protection.